In flat panel electronic displays, light switching elements are arranged in a two dimensional array where the optical state of each element (or pixel) is controlled by at least one transistor. In addition to the transistors used to control the optical state of pixels, transistors are required to form column driving circuit and row driving circuit which control the flow of electrical signals to individual pixels. Since most of the flat panel electronic displays are fabricated on low cost glass substrates, the transistors used to control the optical state and the transistors to form the row and column driving circuits are usually in a thin film form. The active region of conventional thin film transistor is a layer of semiconductor. There are several requirements for a thin film transistor used in high quality electronic display. Firstly the leakage current between drain and source in the Off-state must be small (&lt;10 pA). Secondly, the On-state current at a given voltage (10 volts) must be large (&gt;1 .mu.A). Finally, the threshold voltage must be small and the mobility of charge carriers in the channel must be large.
Conventional TFTs are formed from a thin film of amorphous Si (a-Si). More recently, some TFTs have been fabricated using polycrystalline Si (poly-Si). Thin films of a-Si for transistors are typically deposited by CVD method at substrate temperatures of about 350.degree. C. For deposition of poly-Si films, a LPCVD method is used at substrate temperatures ranging from 500 to 650.degree. C. Hence the power consumption for the deposition of Si films is high. Furthermore, the deposition of uniform Si films on substrates with relatively large dimensions required for the electronic displays is difficult to achieve due to the distribution of molecules in the reaction chamber. Although the mobility of charge carriers in poly-Si is large and in the order of 200 cm.sup.2 /V-sec, the mobility in a-Si is quite small, in the order of 5-10 cm.sup.2 /V-sec. The above mobility values are for Si films prior to the formation of field effect transistors. When a thin film field effect transistor is fabricated with a-Si, the effective mobility of charge carriers is reduced to 0.3-1 cm.sup.2 /V-sec, whereas the effective mobility of carriers in poly-Si is reduced to 25-100 cm.sup.2 /V-sec. The precise values of effective mobility in a-Si and in poly-Si TFTs are determined by the deposition temperature of the a-Si and poly-Si films.
Thin film transistors may be fabricated using CdS or CdSe as an active semiconductor layer. As shown in Table 1, both the Hall mobility, .mu..sub.H, and the effective mobility, .mu..sub.eff, are greater than that of a-Si. Therefore, these two materials are potentially good candidates for forming active layer of thin film transistors. It is also noted that most of thin films of CdS and CdSe reported in literatures are n-type in conduction. Thin film transistors based on CdSe have been fabricated for high brightness LCD panels [see T.P. Brody, IEEE Trans. ED-20, p.995-1001, 1973]. In such a prior art CdSe thin film transistor, thin film of CdSe required for the active layer was deposited by a vacuum thermal evaporation method. Although some success has been demonstrated in the prior art, it has been observed that the reproducibility and uniformity of surface electronic properties of the vacuum deposited large area CdSe films are difficult to control. Because of the difficulty in reproducibility of the surface electronic properties of vacuum deposited CdSe, variation of the threshold voltage of the thin film transistors from one substrate to others is too large to achieve high production yield.
TABLE 1 Energy Gap .mu..sub.H .mu..sub.eff Semiconductors (eV) (cm.sup.2 /V-s) (cm.sup.2 /V-s) CdS 2.4 250 100.about.150 CdSe 1.7 580 20.about.480 a-Si 1.9 5.about.20 0.3.about.1 p-Si 1.12 .about.200 25.about.100 s-Si 1.12 1880 160.about.
Therefore, it would be desirable to develop a method to fabricate thin film transistors where the active semiconductor layer is deposited at low temperatures using a low cost method. The mobility of charge carriers in the deposited films should be as large as possible, preferably greater than 10 cm.sup.2 /V-sec. Furthermore, this method should ideally be capable of depositing semiconductor films on large area substrates without the need of expensive equipment.